1. Field of the Invention
The present invention relates to a screw groove type vacuum pump, and a complex pump and a vacuum pump system both of which include the screw groove type vacuum pump. More specifically, the present invention relates to a screw groove type vacuum pump, a complex pump and a vacuum pump system with which excellent exhaust speed can be attained.
2. Description of the Related Art
Screw groove type vacuum pumps have conventionally been well known. Any of these screw groove type vacuum pumps is provided with a rotor member that rotates and a stator member fixedly arranged so as to be coaxial with the rotor member, and has a screw groove formed on one of a circumferential wall of the rotor member and an opposite wall of the stator member which is opposite to the circumferential wall. The rotor member is rotated to introduce gas from an inlet port into the screw groove and to then transfer the gas along the screw groove, thereby discharging the gas through an outlet port.
In the screw groove type vacuum pump as such, conventionally, a screw sealing technique or the like is applied and the screw groove is designed so as to be rather shallow in order to efficiently transfer with the rotation of the rotor member gas molecules whose pressure is of a viscous flow region while utilizing the viscosity, and to thereby prevent the backward-flow from the outlet port side to the inlet port side.
However, conventional screw groove type vacuum pumps as described above have a problem of slow exhaust speed, for the average free path of gas molecules is large for gas in a molecule flow region and hence it is difficult to introduce the gas into the screw groove.
There has been proposed as a technique for improving the gas exhaust speed a screw groove type vacuum pump in which the screw groove at the inlet port is set deep and the depth of the screw groove is sharply reduced from thereon. In this screw groove type vacuum pump, the intake area of gas that is taken from the inlet port into the screw groove is large making it easy to introduce the gas in the molecule flow region into the screw groove.
On the side downstream of the inlet port, however, the pressure of the gas to be transferred through the screw groove is of an intermediate flow region between the molecule flow region and the viscous flow region, and the average free path of gas molecules is relatively large. For that reason, the gas molecules taken in are reflected by the bottom of the screw groove, or the like, which means that a sufficient exhaust speed cannot be obtained by merely setting the screw groove deeper at the inlet port.
The present inventors have found that, in a screw groove type vacuum pump, the pressure of the gas to be transferred through the screwed groove maintains the pressure of the intermediate flow region between the molecule flow region and the viscous flow region downstream of the inlet port until the gas reaches a certain depth of the pump in the axial direction, and that setting the flow path wider at this certain depth and securing the sealing from thereon result in prevention of reflection and backward flow of gas molecules, prevention of degradation of sealing, improved gas exhaust efficiency and excellent exhaust speed.
The present invention has been made on the basis of the findings as above, and an object of the present invention is therefore to provide a screw groove type vacuum pump, a complex vacuum pump and a vacuum pump system with which excellent exhaust speed can be attained.
In order to achieve the above object, the present invention provides a screw groove type vacuum pump comprising: a rotor member that rotates; a stator member fixedly arranged so as to be coaxial with the rotor member and having an opposite wall that is opposite to a circumferential wall of the rotor member; an inlet port for introducing gas into a space between the circumferential wall of the rotor member and the opposite wall of the stator member; and an outlet port for discharging the gas from the space between the circumferential wall of the rotor member and the opposite wall of the stator member, in which: a screw groove for transferring the gas from the inlet port with the rotation of the rotor member is formed on one of the circumferential wall of the rotor member and the opposite wall of the stator member; the depth of the screw groove at the nearest point to the inlet port is 20 mm or more, or is equal to or larger than xc2xc the diameter, including the screw groove, of one of the circumferential wall of the rotor member and the opposite wall of the stator member, the depth of the screw groove is decreased toward the outlet port side from the inlet port side, and the depth of the screw groove in a region defined by the inlet port and a point on the rotor member which is 40 mm in the axial direction is 80% or more of the depth at the nearest point to the inlet port; and the slant of the screw groove with respect to the radial direction of the rotor member is decreased toward the outlet port side from the inlet port side, and maintains to be 80% or more of the slant at the inlet port until the thread reaches a point on the rotor member which is at least 40 mm in the axial direction.
According to the above screw groove type vacuum pump of the present invention, the depth of the screw groove may be decreased toward the outlet port side, which is downstream of a region defined by the inlet port and the point on the rotor member which is 40 mm in the axial direction, in proportion to the distance in the axial direction. This makes it possible to transfer gas in a viscous flow region with tight sealing.
The slant of the screw groove may be decreased toward the outlet port side, which is downstream of a region defined by the inlet port and the point on the rotor member which is 40 mm in the axial direction, in proportion to the distance in the axial direction. This makes it possible to transfer gas in the viscous flow region with tight sealing.
The slant of the screw groove may be decreased toward the outlet port side, which is downstream of a region defined by the inlet port and the point on the rotor member which is 40 mm in the axial direction, in logarithmic proportion to the distance in the axial direction. This makes it possible to transfer gas in the viscous flow region with tight sealing.
In order to achieve the above object, the present invention also provides a complex vacuum pump including the above screw groove type vacuum pump of the present invention.
In order to achieve the above object, the present invention also provides a vacuum pump system comprising the above screw groove type vacuum pump of the present invention and an auxiliary pump for taking in gas discharged through the outlet port of the screw groove type vacuum pump.
In order to achieve the above object, the present invention also provides a vacuum pump system comprising the above screw groove type vacuum pump of the present invention and an auxiliary pump for taking in gas discharged through the outlet port of the screw groove type vacuum pump that is included in the complex vacuum pump.